Use the proper oil for the specific application. You are comparing apple oil to orange oil here.
The SAE viscosity ratings for engine oil and gear oil are not listed using the same standard. They are designed for different heat ranges, and engine oil needs to function when mixed with gasoline and detergents while withstanding the higher temps in an engine.
To quote the Bel Rey website:
“The most common mistake is the belief that the viscosity of SAE 80W gear oil is much thicker than the viscosity of SAE 10W-40 motor oil. This is absolutely incorrect! A 10W-40 motor oil’s viscosity is actually similar to the viscosity of 80W gear oil, and the viscosity of SAE 10W-30 is similar to 75W gear oil.”
While we all know the benefits of a good engine oil, the advantages of a gear oil are often overlooked because people don’t know the different requirements of lubricating an engine compared to a gearbox. […]
www.belray.com